Many clinical situations in musculoskeletal care, including spinal arthrodesis procedures, require a bone reconstruction strategy to treat contained defects (a hole in a bone), non-contained defects (a missing segment of bone), or fusion across bone generation spaces (where bone would not normally grow). Novel orthopaedic biomaterials that effect guided bone growth into biodegradable polymeric composite scaffolds are candidates to address such requirements, and the goal that has motivated the development of these materials is the augmentation and eventual elimination of current autograft and allograft bone strategies for transplantation into skeletal sites. For the past decade, our laboratory has done extensive work on three-dimensional (3-D) preformed bone scaffolds and transitioned them to clinically relevant large animal models for segmental bone defect repair. The current proposal focuses on the translation of our injectable and moldable bone scaffold work toward initial human use in spinal fusion via three integrated aims.
In Aim 1, we will further optimize members of our suite of biocompatible, biodegradable, and self-crosslinkable fumarate ester polymeric biomaterial networks by inter-crosslinking of poly(propylene fumarate) (PPF) and poly(caprolactone) (PCL) via catalyst-free click chemistry (PPF/PCL). The network will incorporate osteoconductive nano-hydroxyapatite (nano-HA) and degradable hydrogel porogens that encapsulate vascular endothelial growth factor (VEGF) and bone morphogenetic protein-2 (BMP-2). The VEGF-containing hydrogel will degrade faster than the BMP- containing hydrogel to achieve dual, sequential delivery of angiogenic and osteoinductive factors coupled with two-stage porosity generation. The composite PPF/PCL formulations will be optimized separately for injectable and moldable bone scaffolds based on success criteria in rheological and handling properties, mechanical properties, porosity and interconnectivity, degradation rates, and growth factor release profiles.
In Aim 2, we will determine the in vivo effect of the injectable and moldable PPF/PCL scaffold formulations in rabbit interbody and posterolateral spinal fusion models, respectively. Due to the fact that the gold standard, autograft bone, may incur donor site morbidity and can have a suboptimal fusion rate in some situations, spinal fusion is often considered one of the most challenging applications of bone graft substitutes, thus allowing us to critically evaluate the optimized candidate scaffold implant formulations.
In Aim 3, we will assess the bone regeneration performance of PPF/PCL composite scaffolds in a large animal model of clinically relevant human surgical procedures as a translational step toward initial human use. We have selected a sheep unilateral posterior spine pedicle screw instrumented reconstruction model, consisting of either a posterior interbody fusion, a posterolateral intertransverse process fusion, or a combination of both these fusion processes at the same level, utilizing our injectable and moldable scaffold strategies to accomplish these goals.
The biodegradable and self-crosslinkable polymeric composite scaffolds that have been chosen for further development in this project are members of the suite of fumarate ester polymeric biomaterials that our group has designed, developed and translated to human use in other musculoskeletal applications, i.e. segmental nerve defects and (currently in process) segmental bone defects. Our group now plans to work toward new treatment options for spinal arthrodesis surgical procedures using osteoconductive and osteoinductive biomaterial implants to direct fusion of spinal motion segments via bone formation between the vertebrae chosen for fusion to each other. This fusion process will follow either the injection of an in situ polymerizable implant formulation for a vertebral interbody fusion and/or placement of a moldable implant to effect a posterolateral spine fusion, and both application modalities will offer distinct advantages for the performance of the respective fusion types, saving the patients further surgery, expense, and inconvenience.